A great variety of optoelectronic semiconductor structures for converting electrical energy into light, or vice versa, are known.
Key elements of an optoelectronic semiconductor structure meant here comprise an n-type semiconductor material region and a p-type semiconductor material region forming a p-n junction therebetween, and an active region where the actual energy conversion takes place. In light emitters like light emitting diodes (LEDs) and semiconductor lasers, charge carriers injected into the active region by supplying a current through the p-n junction recombine via radiative recombination, thereby emitting light. In structures for energy conversion in the opposite direction, e.g. in photovoltaic cells, charge carriers are generated in the active region via absorption of incident electromagnetic radiation. These carriers are collected to the opposite sides of the p-n junction, thereby generating an electric current in the circuitry into which the structure is connected.
One key design parameter of optoelectronic semiconductor structures is the overall efficiency of the structure. For example, for both light emitting and light receiving structures, the active region taking part in the energy conversion should preferably cover as large a portion of the area of the structure as possible. From operational point of view, one of the most important issues in optimizing the efficiency of an optoelectronic semiconductor structure is the optimization of the current transport between the active region and the p-and n-type semiconductor regions. Particularly in light emitters, the electric current should be spread as uniformly as possible over the entire active region. A non-uniform current density can result in undesired local overheating of the structure at the locations of a high current density. Moreover, non-uniformity of the current density can also increase the non-radiative recombination, thereby decreasing the energy conversion efficiency of the structure. In addition to the uniform current density, optimizing the current transport properties of an optoelectronic semiconductor structure also comprises minimizing the overall resistive losses within the structure.
Prior art optoelectronic semiconductor structures are typically implemented as layered configurations with an active layer superposed between an n-type semiconductor layer and a p-type semiconductor layer. Carrier transport into or from the active region of such optoelectronic structures conventionally takes place via the n-type and p-type layers on opposite sides of the active layer. Forming the contacts to the n-type or p-type layer below the active layer often requires the active region being partly cut off. This is the case e.g. in conventional edge-emitting LED structures having the n-and p-type contacts formed on the same side of the layered structure. Said partial cut-off naturally decreases the portion of the device area covered by the active region. Moreover, due to the internal resistances of the contact layers forming the electrical interfaces of the structure, uniform current distribution necessitates that the contacts lie relatively close to each other and that the sizes of them are limited. In practice, these requirements restrict the maximum area of the active region of high power LED structures to be of the order of 1 mm2.
On the other hand, in vertically emitting structures and in light receiving structures like photovoltaic cells, the surface of the optoelectronic semiconductor structure forming the interface through which light is coupled between the structure and the ambient is partly covered by one of the contact structures. Typically this contact structure is formed as an opaque metallic grid extending essentially over the entire area of the active layer. This kind of contact structure partially obscures the light path between the active layer and the ambient, thereby again decreasing the effective area of the active region.
In both of the contacting schemes described above, the requirements of uniform current density and low resistive losses require perforations in the active layer distributed with typically less than 100 μm separations or contact grids covering several percent of the surface area of the structure.